It is known that the operating characteristics of utility engines (e.g., emissions, power, smoothness, etc.) are influenced by the air to fuel ratio of the fuel. Under high load conditions, a rich mixture is desirable. Under low loads, a lean mixture improves engine emissions performance. Heretofore, control of the air to fuel ratio was accomplished using a carbureted air bleed mechanism which varied the quantity of air delivered to the engine cylinder in relation to the stability of the engine.